Assignment, fit, and theoretical discussion of the ν10 band of acetaldehyde near 509 cm−1

The lowest small-amplitude vibration in acetaldehyde (CH3CHO) is the in-plane aldehyde scissors mode ν10 at 509 cm−1. This mode lies about 175 cm−1 above the top of the barrier to internal rotation of the methyl group and is relatively well separated from other small-amplitude vibrational states (the next fundamental occurring more than 250 cm−1 higher). It thus provides an excellent example of an isolated small-amplitude fundamental (bright state) embedded in a bath of dark states. Since the bath states at these energies are not too dense, and since they arise purely from states of the large-amplitude torsional vibration of the methyl rotor, a detailed spectroscopic analysis of interactions between the bright state and the bath states should be possible. This paper represents the first step toward that goal. We have assigned several thousand transitions in the ν10 band (J ⩽ 28, K ⩽ 12), and have carried out a simultaneous fit of 2400 of these transitions (J ⩽ 15, K ⩽ 9) with over 8100 transitions to the torsional bath state levels. Three vibration-torsion interactions, which give rise to rather global level shifts of the order of 1 cm−1 in the ν10 levels, have been identified and quantitatively fit. A number of vibration-torsion-rotation interactions, which give rise to localized (avoided-crossing) shifts in ν10 have also been determined. The present analysis indicates the need for reliable spectroscopic information on more of the torsional bath states in the immediate vicinity of the ν10 levels. Possible ways of obtaining such information in future studies are considered.